This invention relates to color references, and more particularly relates to monochrome cathode ray tubes (CRTs) as color references, and to a method for producing them.
All colorimetric instruments require calibration against a standard source prior to use. The National Bureau of Standards specifies incandescent lamps as primary sources for such calibration. Arbitrary colors can be achieved through the use of complex filter sets, and stability of the colors can be achieved through the extremely accurate control of the input energy (current and voltage).
However, away from the research laboratory, less costly and cumbersome standards (sometimes called secondary standards) are required. For example, in the manufacturing environment, successful quality control of color requires color standards which are stable, rugged, portable and relatively inexpensive.
A standard color television receiver or test set can be relatively easily adjusted to give an arbitrary color within its color gamut. However, the stability of the color is dependent upon a number of factors, including: registration between the three (red, blue and green) electron guns, the aperture mask and the phosphor pattern on the screen; the relative beam currents in each of the electron guns; and the operating (anode) voltage. One solution to this problem is to eliminate two of the electron guns, the aperture mask and the phosphor pattern, and to produce the desired color standard by physically mixing different phosphors and depositing the resulting blend on the CRT screen. See, for example, U.S. Pat. No. 4,406,971. However, such tubes have been found to be difficult to produce, due primarily to the different physical characteristics of the phosphor powders. For example, when depositing the phosphor mixture by settling from a slurry, different settling rates, as well as packing anomalies, cause a shift in color of the settled deposit from that of the original blend. Thus, considerable trial and error is required to achieve a particular color standard.
Accordingly, it is an object of the invention to produce a color standard which is stable, rugged, portable and relatively inexpensive.
It is also an object of the invention to produce a color standard which uses CRT phosphors but does not depend on the use of phosphor blends.
It is also an object of the invention to produce a color standard from CRT phosphors which is nearly independent of registration and electrical factors.